A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case where silicon is used in a transistor included in a large display device, it is preferable to use amorphous silicon, for which a technique of forming a film over a large substrate is established. In the case where silicon is used in a transistor included in a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate, it is preferable to use polycrystalline silicon, which can form a transistor having high field-effect mobility. As a method for forming polycrystalline silicon, high-temperature heat treatment or laser light treatment which is performed on amorphous silicon has been known.
In recent years, transistors including oxide semiconductors (typically, an In—Ga—Zn oxide) have been actively developed.
Oxide semiconductors have a long history, and it was disclosed to use a crystal In—Ga—Zn oxide for a semiconductor element in 1988 (see Patent Document 1). In 1995, a transistor including an oxide semiconductor was invented, and its electrical characteristics were disclosed (see Patent Document 2).
In 2013, one group reported that an amorphous In—Ga—Zn oxide had an unstable structure in which crystallization is promoted by irradiation with an electron beam (see Non-Patent Document 1). It is also reported that the amorphous In—Ga—Zn oxide formed by the group has no ordering in observation with a high-resolution transmission electron microscope.
In 2014, a transistor using a crystalline In—Ga—Zn oxide that has more excellent electrical characteristics and higher reliability than a transistor using an amorphous In—Ga—Zn oxide was reported (see Non-Patent Document 2, Non-Patent Document 3, and Non-Patent Document 4). Here, it is reported that a crystal boundary is not clearly observed in an In—Ga—Zn oxide including a CAAC-OS (C-Axis Aligned Crystalline Oxide Semiconductor).
A sputtering target with which CAAC-OS having high crystallinity can be formed is disclosed (see Patent Document 3).
Meanwhile, as a kind of a crystalline structure of a polymer, a concept of “paracrystal” is known. A paracrystal is a crystalline structure which seemingly has a trace of crystal lattice but has distortion as compared with an ideal single crystal (see Non-Patent Document 5).